As a luminous flux control member that controls the traveling direction of light emitted from a light source, a luminous flux control member (so-called Fresnel lens) has been conventionally used that has a serrated cross-sectional shape (referred to, hereinafter, as a Fresnel shape) in which a light-incident area and a light-emission area are divided into a plurality of concentric ring-shaped (circular band-shaped) segmented areas. Because this luminous flux control member is suitable for reducing thickness and weight and the like, it is used for various purposes (such as in magnifying glasses and lighting systems).
When this type of luminous flux control member is assembled into a product for illumination, for example, alight source, such as a light-emitting diode (LED), is fixed to the incident area side formed into the Fresnel shape after positioning is performed such that the center axis of light emitted from the light source is coaxially aligned with the optical axis of the luminous flux control member.
In addition, the Fresnel shape of this type of luminous flux control member includes a type having only a refraction surface that refracts light emitted from a light source and a type having a total reflection surface in addition to the refraction surface. The latter type is more advantageous than the former type in terms of efficiently capturing and converging light emitted from the light source (such as an LED) at a wide spread angle.
Specifically, the LED which is an example of a light source emits light having a wide spread angle and light distribution characteristics based on so-called Lambertian distribution. To effectively use this light for illumination and the like, the market requires the light distribution characteristics of the light to be narrowed by the luminous flux control member and the directivity of the light to be improved. Therefore, when the Fresnel shape including only the refraction surface is used to change the light emitted from the LED to light having a narrow-angle light distribution, the amount of change in the traveling direction of light in the incident area of the luminous flux control member is determined only by refraction by the refraction surface. Therefore, no significant change appears in the traveling direction of light transmitted within the luminous flux control member in relation to the traveling direction (original traveling direction) of light at the time of incidence onto the luminous flux control member. Therefore, in particular, light emitted from the LED towards the wide-angle side cannot be sent toward a target irradiated surface by refraction, and spreading of the overall light (light beam) cannot be sufficiently suppressed. On the other hand, when the Fresnel shape including the total reflection surface is used, the light entering the luminous flux control member from the refraction surface can be totally reflected by the total reflection surface and the traveling direction can be significantly changed. Therefore, even light emitted from the LED toward the wide-angle side can be sent towards the irradiated surface side.
Therefore, the Fresnel shape including the total reflection surface is suitable for luminous flux control. However, conversely, the tip portion (top portion) of a projecting section configuring the Fresnel shape becomes sharp due to the total reflection surface, and production tends to be difficult.
For example, when the luminous flux control member is obtained by injection molding, the sharper the tip portion of the projecting section of the Fresnel shape is, the more difficult it is to fill the feature replication surface for the projecting section of the mold with a resin material. Therefore, a molding defect easily occurs in which the edge of the tip portion is not formed due to insufficient filling.
In reflection of this difficulty in production, the sharp Fresnel shape including the total reflection surface requires shape management that is stricter than that for the Fresnel shape including only the refraction surface. Management of height of the projecting section (length from a base end portion to a tip portion) is particularly important.
Here, when the height of the projecting section of the Fresnel shape is measured as part of such shape management, in the instance of a common Fresnel shape including only the refraction surface, the difference between the tip portion and the base end portion of the projecting section of the Fresnel shape can be checked and the height can be accurately determined, by the Fresnel shape being traced while light is shone onto the surface of the Fresnel shape by a general-purpose measuring device, such as a tool microscope or a laser microscope, and reading the position at which focus is achieved.
On the other hand, in the instance of the Fresnel shape including the total reflection surface, the height of the projecting section cannot be accurately measured by a general-purpose measuring device. In the Fresnel shape including the total reflection surface, not only is the tip portion of the projecting section sharp, as described above, in the valley shape formed between adjacent projecting sections, the valley bottom in which the periphery of the base end portion of the projecting section is positioned tends to have a sharp acute angle. Even when light from the measuring device is shone onto the valley bottom formed by such steep inclined surfaces, noise occurs and an accurate focal position (periphery of the base end portion of the projecting section) cannot be read.
Therefore, for shape management of the Fresnel shape including the total reflection surface, measurement of the projecting section may be abandoned and the optical characteristics of the luminous flux control member itself may be evaluated instead. Because the purpose of the luminous flux control member is to achieve desired optical characteristics, such direct evaluation of optical characteristics is a highly reliable product inspection method.
However, a dedicated evaluation device is required for evaluation of optical characteristics. Time is also required for carrying out measurement. Therefore, a problem occurs in that cost increases and inspection efficiency decreases.
Here, in Patent Literature 1, a configuration is disclosed in which a planar section that is perpendicular to the optical axis is provided between projecting sections that are adjacent to each other in the radial direction in the Fresnel shape. A planar section such as this does not change in shape in the height direction and has a certain amount of area. Therefore, optical readout by a general-purpose measuring device can be appropriately performed. Thus, when the planar section is used as reference for the height of the projecting section (in other words, a zero-height position that is at the same height as the base end portion of the projecting section), the height of the projecting section can be simply and accurately measured.    Patent Literature 1: Japanese Patent Laid-open Publication No. 2011-29168